Urine that smells like tuna or fish usually comes from a compound called trimethylamine (TMA) building up in your body and being excreted in your urine. In most cases, TMA buildup is temporary and tied to something you ate, a supplement you’re taking, or a vaginal infection that’s mixing discharge with urine. Less commonly, it signals a genetic condition that prevents your body from processing TMA properly.
How Your Body Normally Handles Fishy Compounds
Every day, bacteria in your gut break down certain nutrients from food and release trimethylamine, a chemical with a strong, fishy odor. Under normal circumstances, TMA is absorbed through your gut wall, travels to the liver, and gets converted into an odorless form called TMAO. Your kidneys then flush TMAO out in your urine without any noticeable smell.
A single liver enzyme, called FMO3, does virtually all of this conversion work. When FMO3 is functioning normally, more than 80% of trimethylamine gets converted to its odorless form before it ever reaches your urine. Problems arise when this enzyme can’t keep up, either because of a genetic issue, because you’ve overwhelmed it with too much TMA at once, or because of hormonal shifts that temporarily reduce its activity.
Foods and Supplements That Trigger the Smell
The most common and least worrying explanation is dietary. Certain foods are especially rich in the precursors that gut bacteria convert into TMA. These include:
- Eggs, which are high in choline
- Saltwater fish like tuna, cod, and other seafood
- Organ meats (liver, kidney)
- Legumes such as soybeans and kidney beans
If you recently ate a large serving of any of these, the temporary surge of TMA can outpace your liver’s ability to convert it all, and some of the smelly compound slips through into your urine. This is especially likely if you’re dehydrated, since concentrated urine amplifies any odor that’s present.
Supplements can also be responsible. L-carnitine supplements, popular among athletes and people with certain heart conditions, have been documented to cause a fishy body and urine odor. In one clinical case, the smell resolved completely once the person stopped taking carnitine. The likely explanation is that excess carnitine saturates the liver’s processing capacity, allowing TMA to accumulate. High-dose choline supplements can do the same thing.
Trimethylaminuria: The Genetic Cause
If the fishy smell is persistent, not tied to any particular meal, and you notice it in your sweat or breath too, you may have a condition called trimethylaminuria (TMAU), sometimes called “fish odor syndrome.” People with TMAU have mutations in the gene that produces the FMO3 enzyme. Without a working version of this enzyme, their bodies can’t convert TMA to its odorless form, so large amounts of the smelly compound are excreted in urine, sweat, and breath.
TMAU is rare but not as uncommon as once thought. In a white British population, roughly 1 in 40,000 people have the severe form. Carrier rates vary significantly by ethnicity: about 0.5 to 1% in British populations, 3.8% in Ecuadorian populations, and as high as 11% in New Guinean populations. Carriers typically don’t have constant symptoms but may notice a fishy smell after eating trigger foods. At a specialized body odor clinic in Philadelphia, 35% of patients referred for unexplained malodor turned out to have trimethylaminuria, suggesting it’s widely underdiagnosed.
Diagnosis involves a urine test that measures the ratio of TMA to TMAO. In healthy individuals, at least 80% of the total trimethylamine in urine is in its odorless TMAO form. People with TMAU have a significantly lower percentage, meaning more of the smelly TMA passes through unchanged.
Bacterial Vaginosis and Urine Odor
If you have a vagina, there’s another common explanation worth considering. Bacterial vaginosis (BV) causes a thin, grayish-white or green discharge with a distinctly fishy smell. Because this discharge can mix with urine when you use the bathroom, many people assume the smell is coming from their pee when it’s actually coming from vaginal secretions.
BV happens when the balance of bacteria in the vagina shifts, allowing certain anaerobic bacteria to overgrow. Along with the fishy odor, you might notice burning during urination or vaginal itching. BV is one of the most common vaginal conditions in women of reproductive age, and it’s treatable. If the tuna-like smell is new, coincides with changes in discharge, or gets stronger after sex, BV is a likely culprit.
Hormonal Shifts and Temporary Enzyme Dips
Even without a genetic mutation, FMO3 enzyme activity fluctuates. Hormonal changes during menstruation, pregnancy, and menopause can temporarily reduce the enzyme’s efficiency, letting more TMA through. Some women notice a fishy urine smell around their period that resolves afterward. This is a mild, transient version of the same mechanism behind TMAU, and it doesn’t indicate a permanent problem.
Liver or Kidney Problems
Because TMA processing happens in the liver, significant liver damage can impair this conversion. Liver disease more commonly produces a musty or sweet odor in urine rather than a distinctly fishy one, but the overlap exists. Kidney problems can also concentrate waste products and intensify urine odors of all kinds. If the smell persists and comes with other symptoms like dark urine, yellowing skin, swelling, or fatigue, liver or kidney involvement is worth investigating.
What You Can Do About It
If the smell appeared suddenly, start with the simplest fixes. Drink more water to dilute your urine. Cut back on choline-heavy foods (eggs, organ meats, certain fish) for a few days and see if the smell resolves. If you’re taking L-carnitine or choline supplements, try stopping them temporarily.
If the smell is persistent, keeps coming back, or shows up in your sweat and breath as well, ask your doctor about a urine test for TMA and TMAO levels. This is a straightforward test, though not every lab offers it. People diagnosed with TMAU often manage symptoms by limiting dietary triggers, and some respond to riboflavin (vitamin B2) supplementation, which can enhance whatever residual FMO3 activity they have.
For anyone noticing the smell alongside vaginal discharge or irritation, a simple exam and swab test can confirm or rule out BV quickly. Treatment typically clears both the infection and the odor within a week.